The formation of certain papers involves the retention of fine particles including fiber fines and inorganic fillers in the pulp slurry during the papermaking process such that the fillers impart desired properties to the resulting paper. During the production of fine alkaline papers, for instance, calcium carbonate is used as a filler to impart desired optical properties such as brightness and opacity. In addition, since calcium carbonate is significantly less expensive than pulp fibers, use of calcium carbonate as a filler reduces the cost of producing paper. To improve fine particles retention in the paper retention aids are commonly used. Retention aids are usually high molecular weight acrylamide copolymers in which the comonomer can be anionic, cationic or nonionic. These water soluble retention aids are essentially flocculants by nature.
The formation of paper from a pulp slurry also involves the drainage of process water on a continuously moving wire screen, dewatering by pressing wet webs between wet felts and drying on a series of heated drums. The rate of paper production depends on how efficiently the water is removed from the web during this process without adversely affecting the formation or physical properties of the sheet. Drainage aids are often used to increase the rate of dewatering. Drainage aids are usually low molecular weight water soluble polymers or resins that have a high cationic charge density. It is common practice to use both a drainage aid and a retention aid during the production of paper such as alkaline fine paper.
It has been found unexpectedly that the retention of fillers in the sheet can be increased during the papermaking process by adding to a filler-containing alkaline fine paper slurry resins known to provide wet and/or dry strength to unfilled paper. Specifically, an aminopolyamide-epichlorohydrin (APAE) resin and a glyoxylated acrylamide-diallyldimethylammonium chloride (GPA) resin, preferably in the form of a mixed resin solution, are added to the slurry during the papermaking process. The resins are added in an amount and at a rate sufficient to increase the retention of the filler in the paper and the drainage of the process water from the slurry. By so doing, the speed of operation of a papermaking machine can be substantially increased.
U.S. Pat. No. 5,674,362 discloses the use of the same resins in a recycle pulp slurry to provide both wet and dry strength. The slurries used in this invention differ from recycle pulp slurries due to the presence of the filler material. Furthermore, the slurries differ because alkaline fine paper is made using bleached pulp, usually predominantly virgin pulp with little or no recycle pulp. Also, the papers produced hereby have been found to exhibit no significant increase in dry strength as compared to paper produced in the absence of the two resins.
It is an object of this invention to develop a method for improving retention of fillers in the pulp slurry and drainage of process water from the slurry during a papermaking process.
It is a further object to increase the speed of an alkaline fine paper papermaking machine.
These and still further objects will be apparent from the following detailed disclosure of the present invention.